Because some bright individual at FDev thought 2 tons worth of computers were needed to do the job of a smartphone.
It's the 34th century, sure...be we do enjoy our 1950s computing technology.
That sounds about right.
In comparison:
2 tons of hardware to control
ONE limpet! (watches it crash into the next rock)
8 tons of hardware to control two limpets. (hang on, isn't double to two 4 and not 8?)
32 tons of hardware to control and coordinate 3 limpets! (Even Nasa's computer in 69 weighed less than that and it landed a moon lander).
and to top it all off:
128 tons of hardware to control 4 limpets! 4 measly limpets, that happily crash into an asteroid, or start buzzing around the rock, not able to get to a chunk until the rock spins enough for them to get smashed.
In comparison, the Apollo guidance computer weighed a whooping 32kg.
en.wikipedia.org
So if a 128t computer cannot control 4 limpets, to avoid rocks, i think the odds of a 2t computer being able to get us out of the mail slot (make that thing bigger!), into SC, around a star and to our destination, drop out of SC and land us at a station, are fairly low. and by low I mean SUPER low.
Consider your ship a limpet. A highly expendable limpet.